Conventionally, a wire-bonding method has been used for mounting an LED (light-emitting diode) chip on a ceramic substrate (mounting substrate) or the like. However, the wire-bonding method has a problem, for example, that miniaturization is difficult because an area for connecting a wire is required on the ceramic substrate. Moreover, a shadow of the wire is formed when light emitted from the LED chip is applied to the wire, and this results in decreasing a luminous efficiency. Thus, in recent years, a method known as a flip-chip mounting method, in which an electrode of the LED chip is mounted so as to face the ceramic substrate, has been proposed.
FIG. 5 is a cross-sectional view of an LED chip 1 in which a sapphire substrate is employed. The LED chip 1 includes a sapphire substrate 2, an n-type nitride semiconductor layer 3, a p-type nitride semiconductor layer 4, a negative electrode 5, and a positive electrode 6. By impressing a forward voltage between the negative electrode 5 and the positive electrode 6, an electron and a hole are combined so as to emit light in an active layer 4a near a boundary between the n-type nitride semiconductor layer 3 and the p-type nitride semiconductor layer 4.
Whereas the negative electrode 5 is formed on an upper surface of the n-type nitride semiconductor layer 3, the positive electrode 6 is formed on an upper surface of the p-type nitride semiconductor layer 4, which is laminated on the upper surface of the n-type nitride semiconductor layer 3. As such, there is a difference H in height between an upper surface of the negative electrode 5 and an upper surface of the positive electrode 6. As a result, when the LED chip 1 is mounted on the ceramic substrate in the flip-chip mounting method, there is a problem as described below.
FIG. 6 is a cross-sectional view illustrating a state in which the LED chip 1 is to be mounted on a ceramic substrate 9 via two bumps: a bump 10a and a bump 10b. A size of the bump 10a and a size of the bump 10b are the same. The bump 10a is formed on the negative electrode 5 of the LED chip 1, and the bump 10b is formed on the positive electrode 6 of the LED chip 1. As described above, there is a difference in height between the negative electrode 5 and the positive electrode 6 of the LED chip 1. Because of this, a height of a surface of the bump 10a and a height of a surface of the bump 10b are different from each other. Therefore, when the LED chip 1 is flip-chip mounted on the ceramic substrate 9 in a state as illustrated in FIG. 6, the LED chip 1 is inclined with respect to the ceramic substrate 9. As a result, a directivity of light emitted from the LED chip 1 decreases.
FIG. 7 is a graph showing a relation between a load applied to the entire LED chip 1 and an amount of deformation of each of the bump 10a and the bump 10b. For example, in a case of deforming the bump 10b by 10 μm, a load of approximately 14 N/mm2 needs to be applied to the entire LED chip 1. On the other hand, in a case of deforming the bump 10a by 10 μm, a load of approximately 20 N/mm2 needs to be applied to the entire LED chip 1. Thus, in a case of deforming the bump 10a to the same extent as the bump 10b, a load to be applied to the entire LED chip 1 increases approximately 1.5 times. Therefore, when the bump 10a and the bump 10b are compressed in order to eliminate a difference in height between the bump 10a and the bump 10b, a large load is applied to an element in a lower layer of the positive electrode 6. This causes the element to be damaged.
In order to solve the problem, for example, Patent Literature 1 describes a method for (i) forming balls (bumps) by melting an end section of a metal wire at an end section of a capillary, (ii) causing the balls to be adhered to an electrode surface, and (iii) subsequently causing the end section of the capillary to slide on the balls substantially in parallel with respect to the electrode surface so that the balls have the same height.
Japanese Patent Application Publication, Tokukai, No. 2002-118137 A (Publication Date: Apr. 19, 2002)